Aluminum polishing process



United States Patent U.S. Cl. 156-21 5 Claims ABSTRACT OF THE DISCLOSUREThe polishing of decorative and architectural aluminum alloy extrusionsin an etchant solution results in removal of an extremely limited amountof material, of the order of .0005 in. to .001 in., for example. Hencethe polishing process has been and is herein referred to as etching.

This invention relates to an improved process for etching aluminumalloys and, more particularly, relates to a process, as aforesaid, inwhich the etched products have a substantially uniform gloss finish frompiece to piec and in which the etchant bath has a much longer usefullife than it has heretofore been possible to achieve.

It is well known to etch aluminum alloy articles by immersing sucharticles in hot aqueous alkali solutions, e.g., a sodium hydroxidesolution. The etched article is then treated with a suitable acid toremove smut, following which the etched article can be rinsed and driedor processed further such as in anodizing.

The following discussion with respect to aluminum alloy extrusions shallhave reference to alloys of the following range of compositions:

Si, percent .2.8 Fe, percent (max.) .4 Cu, percent (max.) .2

The following discussion with respect to aluminum alloy sheets shallhave reference to alloys of the following range of compositions:

Si-l-Fe, percent (max.)

Cu, percent (max.) .1

Mg, percent 2.2-2.8 15-.25

Cr All other elements, except aluminum, .l% max. for each element.

Al Balance A prime factor limiting the useful life of the etchant bathfor etching aluminum extrusions of the composition aforesaid fordecorative or architectural purposes is that sooner or latergalvanizing? of the surfaces of the etched products will occur.Galvanizing as used herein refers to the phenomena according to whichthe grain located near the surface of the aluminum extrusion becomesrelatively highly visible.

It may be noted here that the galvanizing problem appears to be limitedto aluminum extrusions because aluminum alloy sheets, for example, as aresult of their smaller grain size and other metallurgical factors, donot appear to be subject to galvanizing. Moreover, aluminum alloycastings, although having a larger grain size are subject to havingnonuniform composition and porosity and therefore have only limited usefor decorative or architectural purposes. The previous theory has beenthat 3,464,870 Patented Sept. 2, 1969 galvanizing is the result ofaluminum build-up in the etchant bath. It has been considered that themaximum aluminum concentration in the etchant bath that can betolerated, and still entirely avoid galvanizing, is approximately 25 g.of dissolved aluminum per liter.

While it has long been desirable that aluminum alloy extrusions andsheets of the compositions mentioned above, intended for architecturalpurposes, have a surface finish of uniform gloss, it has been diificultheretofore to achieve such a finish which is uniform from piece topiece. This is largely due to the fact that as the amount of dissolvedaluminum in the bath increases, the activity of the bath changes. Inworking with this problem, we have discovered that etched finishes ofsubstantially uniform gloss can be obtained on aluminum alloy extrusionsand sheets, of the compositions mentioned above, provided that theetchant bath contains at least about 50 gms. of dissolved aluminum perliter and further that such etched finishes Will remain substantiallyuniform from piece to piece so long as the dissolved aluminumconcentration in the bath is substantially constant during an extendedproduction run. Uniform gloss is achieved using production baths havinga substantially constant dissolved aluminum concentration so long assame is within the range of 50 to or more gms. per liter.

The gloss of aluminum extrusions can be measured by a suitable glossmeter, such as one manufactured by Photovolt Corporation. Such metersare graduated to indicate the gloss of the surface measured. Readingscan be based on a perfect mirror finish having a reading of 1000. Thealuminum alloy extrusions and sheets produced by the present inventionunder typical conditions have a gloss measured at 45 to the surface ofabout when the dissolved aluminum concentration in the bath is 50 g. perliter and they have a gloss of about 200 when the dissolved aluminumconcentration is about 80 g. per liter. For high quality, uniform glosswork, the gloss reading should be maintained within a range of not morethan about :20% of the median gloss reading.

In carrying out the etching process using an etchant bath containing atleast 30 g. of dissolved aluminum per liter, the galvanizing problem onaluminum extrusions referredto above has been encountered and this hasnecessitated frequent changes of the etchant bath. This represents asubstantial cost item not only because of the cost of the etchantmaterial itself but also because of the time and the labor cost involvedin changing the bath.

While it would appear, from the prior knowledge, that the problems ofeliminating galvanizing and at the same time achieving a uniform glossfinish on aluminum alloy extrusions of the composition referred to aboveusing a long-lived etchant bath involved mutually inconsistentsolutions, we have discovered that these problems canv be solvedsimultaneously if there is added to the etchant bath a suitable materialfor insuring the presence of sulfide ions therein. We have discoveredthat the cause of the galvanizing problem in etchant baths having a highdissolved aluminum concentration is the dissolved zinc which alsobecomes present in the etchant bath. As little as four or five parts permillion of zinc in the etchant bath containing a high dissolved aluminumconcentration is capable of causing galvanizing. The aluminum alloy thatis being etched is the major source of the dissolved zinc, althoughother possible sources include the make-up water supplied, andimpurities in the proprietary etchant. A typical value for zinc which ispresent in 6063 aluminum alloy is'.02%. It has been found that thepresence of small amounts of sulfide ions in the etchant bath willprecipitate the zinc sulfide and that such substantially eliminates theproblem of galvanizing in etchant baths having a high dissolved aluminumconcentration.

According to the invention,'there is provided a method of etching analuminum alloy product which comprises treating the product with analkali etching solution having dissolved therein at least about 50 g. ofaluminum per liter and also having present therein a positiveconcentration of sulfide ions. For uniform gloss products of either theextrusion or sheet type having the compositions referred to above, theminimum dissolved aluminum concentration is 50 g. per liter. The etchedsurface of an extrusion thus treated is free of galvanizing effectscaused by the etch bath and is smooth and has a uniform gloss as thebath ages further. This gloss will be substantially uniform for allaluminum alloy extrusions and sheets treated by the same type of bathunder similar conditions of time, temperature and alkali concentrationin the bath. Moreover, the useful life of the bath will be of indefiniteduration, provided that suitable amounts of make-up chemicals are addedthereto from time to time to maintain the desired concentrations of suchchemicals.

The method according to the invention can be carried out on aluminumsheets or extrusions of the compositions referred to above, it beingnoted here that the invention does not pertain to the treatment of otheraluminum alloys. An aluminum alloy which provides especially goodresults is the commercially available alloy 6063. However, other 6063type aluminum alloys can be etched according to the invention to obtainsimilar improved results.

The etching treatment is carried out in a hot aqueous alkali solutioncontaining, for example, sodium or potas sium hydroxide, or otheralkaline materials, sodium hydroxide, being preferred. The alkaliconcentration of the etchant bath is not critical and such concentrationcan range from 2 oz. alkali/ gallon of Water to oz. alkali/ gallon ofwater. The alkali solution also can contain various conventionalnoncritical additives, such as sodium gluconate, which are added toimprove the etching effect, prevent scale formation, etc.

The alkali solution must contain at least about 50 g. of dissolvedaluminum per liter in order to achieved the desired uniform gloss on thesurface of the treated aluminum alloy products. Lesser amounts ofdissolved aluminum in the etchant bath will cause variations in thegloss of the etched extrusion which are diflicult to control. Themaximum dissolved aluminum concentration in the etching bath is limitedonly by the ability of the etchant solution to retain the aluminum insolution. This is affected by a variety of factors, includingparticularly the temperature of the solution. It has been found thatwhen a suitable temperature is maintained, the concentration of thedissolved aluminum in the etchant bath can be as high as about 90-105 g.per liter Without difficulty, and even higher dissolved aluminumconcentrations can be used in appropriate cases.

As indicated above, there will be some variation in the gloss leveldepending on the concentration of the dissolved aluminum in the etchantbath. During start up with a fresh etchant bath, it will be advisable tobuild up the concentration of the dissolved aluminum to the desiredlevel before etching the parts which require a uniform gloss. This canbe done by etching scrap aluminum pieces, by using the bath to etchpieces which do not require a uniform gloss, or by employing part of anexisting bath as make up for the new bath.

While aluminum is continuously being dissolved in the etchant bath asthe extrusions are etched so that the aluminum concentration of the bathtends to rise, this presents no serious problem because some of thedissolved aluminum is continually being removed from the etchant bathwith the etched products. That is, a certain amount of the etchant bathwill adhere to the etched extrusion or will collect in pockets thereinand the same will be removed from the remainder of the bath along withthe extrusion. This removal of the part of the etchant is commonlytermed drag out. It is customary to add fresh etching solution to thebath from time to time in order to make up for the part that is removedfrom the bath with the etched product and to make up for that consumedin one chemical reaction. Since the make-up etching solution is free ofaluminum, the aluminum concentration in the etchant bath is therebymaintained within fairly constant limits because the rate at which thealuminum is dissolved from the etched products becomes more or lessequal to the amount of dissolved aluminum that is removed from the bathas a result of the drag out.

The temperature of the etchant bath may range from F. to 180 F. It hasbeen found satisfactory to use etching times of from 1 to 30 minutes.

A positive sulfide ion concentration is maintained in the etchant bathat all times in order to precipitate Zinc as zinc sulfide. Because ofthe low solubility of zinc sulfide in alkaline aqueous solutions, it ispossible to main tain the residual zinc concentration in the bath at asufficiently low level that galvanizing does not occur. The sulfide ionconcentration is continuously maintained at a level of from four toeight gms. per liter (calculated as sodium sulfide) in order to assureprecipitation of the zinc. A higher level of the sulfide concentrationcan be maintained, if desired, but such is not necessary. Similarly,lower concentrations of the sulfide ions can be used, provided that atall times there is a positive sulfide concentration in the etchant bath,but lower amounts ncrease the risk that galvanizing will occur.Desirably the sulfide material is added to the bath from time to time tomake up for the part thereof which precipitates with the zinc and otheralkali insoluble metallic sulfides and, also, the part that leaves thebath with the etched product as a result of the drag out. The sulfidematerial will be required at a rate of one to ten gms. for each 100 gms.of the make-up etchant material added to the bath. Examples of thesulfide ions supplying material that can be used include hydrogensulfide, alkali sulfides and polysulfides, sodium sulfide beingpreferred. These materials may be employed individually or as a mixtureof two or more of these materials.

It will be understood that the so-called smut remaining on the surfaceof the extrusion after etching can be removed in any suitable convenientfashion. As this procedure is conventional and well known and forms nopart of the inventionya detailed description thereof is believedunnecessary.

Gloss differences can be minimized by anodizing or other aftertreatment.

Examples (I) An extrusion of 6063 alloy was etched for 10 minutes at F.in an aqueous solution containing 5 oz. per gallon of NaOH and 2 gms.per liter per gallon of sorbitol. No dissolved aluminum was present inthe solution. The extrusion was then rinsed in tap water and desmuttedat ambient temperature in 30% by weight nitric acid. No galvanizingoccurred and the gloss measurement was 215.

(II) Another sample of the same extrusion was etched for 10 minutes at130 F. in an aqueous solution containing the same etchant as above butwith 20 gms. per liter dissolved aluminum. The extrusion was then rinsedin tap water and desmutted at ambient temperature in 30% by weightnitric acid. No galvanizing occurred and the gloss measurement was 120.

(III) A third sample of the same extrusion was etched for 10 minutes at130 F. in an aqueous solution containing the same etchant and 40 gms.per liter dissolved aluminum. The extrusion was then rinsed in tap waterand desmutted at ambient temperature in 30% by weight nitric acid.Galvanizing occurred which resulted in scrap material.

(IV) A fourth sample of the same extrusion was etched for 10 minutes at130 F. in an aqueous solution containing the same etchant, 40 gms. perliter dissolved aluminum, and 4 gms. per liter sodium sulfide. Thissolution was the same as that used in Example III with 4 gms. per litersodium sulfide added. The extrusion was then rinsed in tap water anddesmutted at ambient temperature in 30% by weight nitric acid. Nogalvanizing occurred and the gloss measurement was 150.

(V) A fifth sample of the same extrusion was etched for minutes at 130F. in a duplicate of the solution used in Example II, which had added toit 6 parts per million zinc. The extrusion was then rinsed in tap waterand desmutted at ambient temperature in 30% by weight nitric acid.Because of galvanizing, the sample was rejected.

(VI) A sixth sample of the same extrusion was etched for 10 minutes at130 F. in the same solution used in Example V, which had added to it 4gms. per liter sodium sulfide. The extrusion was then rinsed in tapwater and desmutted at ambient temperature in 30% by weight nitric acid.No galvanizing occurred and the gloss measurement was 105 as measured inthe same manner as sample I.

(VII) A seventh sample of the same extrusion was etched for 10 minutesat 130 F. in an aqueous solution containing 5 oz. per gallon of NaOH, 2gms. per liter sorbitol, 50 gms. per liter dissolved aluminum, and 4gms. per liter dissolved Na S. The extrusion was then rinsed in tapwater and desmutted in 30% by weight HNO acid. No galvanizing occurredand the gloss measurement was 170 as measured in the same manner assample I.

(VHI) Repeat of VII using 70 gms. per liter aluminum. Gloss 200.

(IX) Repeat of VII using 90 gms. per liter aluminum. Gloss 220.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of polishing aluminum alloy extrusions which contain minutetraces of zinc for decorative or architectural purposes in an etchingsolution to produce a uniform gloss finish thereon, to substantiallyprevent galvanizing of the surfaces thereof and to substantially extendthe useful life of said etching solution, which includes immersing saidextrusions in an alkali etching solution having dissolved therein atleast about grams per liter of aluminum and having present thereinsulfide ions sufficient to limit the presence of zinc to less than fiveparts per million.

2. A method according to claim 1, in which the maximum aluminumconcentration in the etching solution is about gms. per liter.

3. A method according to claim 1, in which the sulfide ions are obtainedby dissolving in the etching solution materials selected from the groupconsisting of sulfides and polysulfides of alkali metals.

4. A method according to claim 1, in which the sulfide ions are obtainedby dissolving sodium sulfide in the etching solution.

5. A method according to claim 1, in which the temperature of theetching solution is maintained between 100 F. and F. and the aluminumalloy extrusions are immersed in the etching solution for from 1 to 30minutes.

References Cited UNITED STATES PATENTS 3,300,349 1/1967 Tershin et al.156-22 3,356,550 12/1967 Stiflier et al. 15622 JACOB H. STEINBERG,Primary Examiner

